A team of physicists from the University of Innsbruck and Harvard University have developed a technique for generating laser light that eschews the use of mirrors. The researchers' work demonstrated that quantum emitters spaced at subwavelength distances can constructively synchronize their photon emissions to produce a bright, very narrow-band light beam, even without the use of an optical cavity. The research combines the theory of light-matter interactions with advanced numerical methods to explore how large atomic ensembles behave collectively and emit coherent radiation. The results suggest that with ongoing progress in the field, mirrorless lasing could soon move from theoretical prediction to experimental realization. Lasers typically use mirrors to bounce light back and forth, stimulating coherent emission from excited atoms or molecules, and thus light amplification. In the mirrorless concept, the atoms interact directly through their own electromagnetic dipole fields, given that interatomic spacing is smaller than the emitted light’s wavelength. When the system is pumped with enough energy, these interactions cause the emitters to lock together and radiate collectively in a phenomenon called super-radiant emission. Passive emitters can significantly enhance the emission of light due to strong dipole-dipole interatomic interactions, and simultaneously, their presence allows for substantial spectral narrowing of the emitted radiation. Courtesy of the University of Innsbruck. The team found that this collective emission generates light that is both highly directional and spectrally pure, with a single narrow spectral line, in cases where only a fraction of emitters are excited by incoherent light and the rest of the atoms remain unpumped. Since this passive emitter fraction is not broadened by the external light or power broadening, it effectively acts as an optical resonator for the active emitters, in analogy with a conventional laser where the optical resonator and the gain medium are separate physical entities. “The atoms synchronize their emission and above a certain threshold start to shine light collectively or in unison with each other,” said Anna Bychek, from the University of Innsbruck. “There are still many questions to be studied in future work, but it is clear that atoms build their own feedback mechanism and frequency selection via dipole-dipole interaction in free space.” Beyond its conceptual significance, this discovery points to a new class of ultra-compact light sources for nanophotonics and precision measurements. Because the emission frequency is determined primarily by the atoms themselves, such systems could provide exceptionally stable optical references for quantum sensors, clocks, or on-chip devices. This research was published in Physical Review Journals (www.doi.org/10.1103/rbs2-2pd5).